If you already installed the proprietary Nvidia driver, then remove it first:

# pacman -Rdds nvidia nvidia-utils nvidia-libgl

Be sure to also delete the /etc/X11/xorg.conf file that the Nvidia driver created (or undo the changes), or else X will fail to properly load the Nouveau driver.

Installation

Before proceeding, figure out your card's codename (a more detailed list is available on Wikipedia) and have a look at the feature matrix to see what features are supported for your graphics card. Also make sure you have Xorg properly installed.

KMS

Kernel Mode Setting (KMS) is required by the Nouveau driver. As the system boots, the resolution will likely change when KMS initializes the display driver. Simply installing the Nouveau driver should be enough to get the system to recognize and initialize it in "Late start" mode (see below). See the Nouveau KernelModeSetting page for more details.

Note: Users may prefer the early start method as it does not cause the annoying resolution change part way through the boot process.

Late start

This method will start the KMS after the other kernel modules are loaded. You will see the text "Loading modules" and the size of the text may change, possibly with an undesirable flicker.

Early start

This method will start the KMS as early as possible in the boot process, when the initramfs is loaded.

To do this, add nouveau to the MODULES array in /etc/mkinitcpio.conf:

MODULES="... nouveau ..."

If you are using a custom EDID file, you should embed it into initramfs as well:

/etc/mkinitcpio.conf

FILES="/lib/firmware/edid/your_edid.bin"

Re-generate the initial ramdisk image:

# mkinitcpio -p <kernel preset; e.g. linux>

If you're experiencing troubles with Nouveau leading to rebuild nouveau-drm several times for testing purposes, do not add nouveau to the initramfs. It is too easy to forget to rebuild the initramfs and it will just make any testing harder. Just use "Late start" until you are confident the system is stable. There might be additional problems with initramfs if you need a custom firmware (generally not advised).

Tips and tricks

Keep NVIDIA driver installed

If you want to keep the proprietary NVIDIA driver installed, but want to use the Nouveau driver, comment out nouveau blacklisting in /etc/modprobe.d/nouveau_blacklist.conf modifying it as follows:

#blacklist nouveau

And tell Xorg to load nouveau instead of nvidia by creating the file /etc/X11/xorg.conf.d/20-nouveau.conf with the following content:

Section "Device"
Identifier "Nvidia card"
Driver "nouveau"
EndSection

Tip: You can use these scripts if you are switching between open and closed drivers often.

If you already used the NVIDIA driver, and want to test Nouveau without reboot, make sure the 'nvidia' module is no longer loaded:

# rmmod nvidia

Then load the 'nouveau' module:

# modprobe nouveau

And check that it loaded fine by looking at kernel messages:

$ dmesg

Installing the latest development packages

You may install the latest -git packages, through AUR:

You can use mesa-gitAUR which will allow the installation of the latest Mesa (including the latest DRI driver).

Tear-free compositing

Dual Head

Nouveau supports the xrandr extension for modesetting and multiple monitors. See the RandR12 page for tutorials.

Here is a full sample /etc/X11/xorg.conf.d/20-nouveau.conf above for running 2 monitors in dual head mode. You may prefer to use a graphical tool to configure monitors like GNOME Control Center's Display panel (gnome-control-center display).